Circuit for rejection of contact bounce



March 31, 1970 J. c. BOATMAN CIRCUIT FOR REJECTION OF CONTACT BOUNCE Filed April 12, 1967 I I I I I I I L l/vvs/v'rm p .J. 1:. EUFITMFIN (9 4% WM JTTUQNEH United States Patent 3,504,198 CIRCUIT FOR REJECTION 0F CONTACT BOUNCE James C. Boatman, Santa Maria, Calif., assignor to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York Filed Apr. 12, 1967, Ser. No. 630,439 Int. Cl. H03k 17/28, 17/74 US. Cl. 307259 9 Claims ABSTRACT OF THE DISCLOSURE The circuit produces only one output pulse on an output terminal for each closure of a pair of contacts even though the contacts bounce and have several engagements during closure. The circuit includes a capacitor and a diode serially connected between one of the contacts and the output terminal. The capacitor, the diode, and the contacts are biased such that upon closure of the contacts, the diode is momentarily rendered conductive only during the first engagement of the contacts to produce an output pulse on the terminal.

GOVERNMENT CONTRACT The invention herein claimed was made in the course of or under contract with the United States Air Force.

BACKGROUND OF THE INVENTION Field of the invention Description of the prior art The above problem has long been recognized in the art and various means have been proposed for eliminating the effect of contact bounce or chatter. For example, filter networks and monostable circuits have been used. However, the prior art apparatus is characterized by either its inability to completely eliminate the effect of contact bounce, its complexity, or its inability to respond rapidly to the closing of the contacts.

SUMMARY OF THE INVENTION It is an object of this invention to produce a circuit which eliminates the eifects of contact bounce.

With this and other objects in view, the present invention contemplates a diode and a capacitor serially con nected between one of a pair of contacts and an output terminal. The diode, the capacitor, and the contacts are biased in such a manner that the diode is rendered momentarily conductive only during the time of the initial engagement of the contacts to produce an output pulse on the terminal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a circuit for eliminating the effect of bouncing of a pair of contacts.

FIG. 2 shows a circuit connecting a plurality of contacts to a terminal wherein the eflect of bouncing of a pair of contacts is eliminated.

DETAILED DESCRIPTION Referring first to FIG. 1, there is shown a circuit for connecting a pair of contacts 10 and 11 to an output ter- 'ice minal 12 wherein only one output pulse is applied to the output terminal 12 for each closure of the contacts 10 and 11 even though the contacts 10 and 11 bounce and produce serveral pulses during each closure. The contacts may be closed by the magnetic field of a relay, a cam, a key, or any other device for bringing contacts together. The circuit includes a capacitor 14 and a diode 15 serially connected between the contact 10 and the output terminal 12. A bias voltage of 20 volts is applied through a resistor 17 to the junction 18 of the contact 10 and the capacitor 14. Similarly, a bias voltage of -20 volts is applied through a resistor 20 to the junction 21 between the capacitor 14 and the diode 15.

The voltage on the junction 21 is limited to a range between -16 volts and approximately zero or ground voltage by a diode 22 connected between the junction 21 and a -16 volt source and by a diode 27 connected between the junction 21 and ground. When the contacts 10 and 11 open, the junction 18 is charged by current through resistor 17 to a voltage of 20 volts and the junction 21 is charged by current through resistor 20 to a voltage of 16 volts. Thus, the junction 21 is 4 volts positive with respect to the junction 18 and the capacitor 14 is charged in a first direction to 4 volts.

A resistor 24 is connected between the output terminal 12 and a -20 volt source. The voltage on the output terminal 12 is limited to a voltage above 3.5 volts by a diode 25 connected between the output terminal 12 and a -3.5 volt source. When the contacts 10 and 11 are open the voltage on the output terminal is 3.5 volts. Thus, the output terminal 12 is 12.5 volts positive with respect to the junction 21 and the diode 15 is negatively biased to render it non-conductive.

When the contacts 10 and 11 close, their initial engagement raises the voltage on the junction 18 to ground voltage by connecting the junction 18 to ground through contact 11. Due to the charge on the capacitor 14, the junction 21 rises to a voltage greater than zero volt and less than +4 volts to render the diode 15 conductive and apply an output pulse to the terminal 12. The peak voltage on junction 21 depends upon such factor as lead inductance and resistance and the forward conductivity of the diode 27. The capacitor 14 very rapidly discharges through the limiting diode 27, and the resistors 20 and 24 until the voltage on junction 21 reaches ground voltage at which time the diode 27 becomes non-conducting. The capacitor then charges in a second direction through resistors 20 and 24 until the voltage on junction 21 reaches -3.5 volts at which time the diode 15 becomes nonconductive. Then the capacitor 14 continues to charge in the second direction through the resistor 20 until the junction 21 reaches its lower limiting voltage of l6 volts.

The contacts 10 and 11, during closure, initially engage for a period of time until the elasticity of the contacts causes the recoil of the contacts sufiiciently apart to open the circuit between the contacts. The values of the resistors 20' and 24, the diode 27, and the capacitor 14 are chosen so that capacitor 14, originally charged in the first direction, discharges to zero volt and then charges in the second direction sufficiently to drop the voltage on junction 21 below 3.5 volts during the period of the first enagement of the contacts 10 and 11 during closure. Thus, the voltage on the junction 18 will be more than 3.5 volts positive with respect to the junction 21. For the voltages recited above, the discharge and charge of capacitor 14 in the second direction will be sufiicient if the value of the capacitor 14 and the resistor 20 are chosen to have a series time constant equal to or less than the period of the initial engagement of the contacts 10 and 11.

When the contacts 10 and 11 bounce and momentarily separate, a current through the resistor 17 tends to discharge and charge the capacitor 14 back in the first direction to lower the voltage on junction 18 toward -2() volts and to bring about the initial voltage conditions of the circuit. The value of the resistor 17 is chosen so that the voltage on junction 18 remains greater than 3.5 volts positive with respect to junction 21 during the momentary opening of the contacts and 11 due to bouncing. Thus, in subsequent bouncing of the contacts 10 and 11, the voltage on junction 21 remains below -3.5 volts and the diode 15 remains non-conductive. However, the value of the resistor 17 must be small enough to insure recharging of the capacitor 14 in the first direction between closures of the contacts 10 and 11. An example of an acceptable value for the resistor 17 in conjunction with the capacitor 14 is for the resistor 17 and capacitor to have a series time constant of approximately 5 times the total period over which the contacts 10 and 11 bounce.

Referring now to FIG. 2, it may be desirable to connect additional contacts, such as contacts 28 and 29, to the output terminal 12. Circuit 31 of FIG. 2 is the same as that shown within the dashed box 31 of FIG. 2 which contains the resistors 17 and 20, the capacitor 14, and the diode 22. Circuits 32 and 33, connected to one of the contacts in each of the contacts 28 and 29, are similar to the circuit 31 and operate in a similar manner. The circuits 31, 32 and 33 are connected by respective isolating diodes 35, 36 and 37 to a common junction 40 which is connected to the diode 15. A bias voltage is applied to the junction 40 by a resistor 39 connected to a voltage source. The discharge and charge path for the capacitor 14 in the second direction now includes the resistors 20, 24 and 39 and the diode 27. The time constant of this discharge and charge path in the second direction is calculated so that the voltage on the terminal 40 drops below 3.5 volts during the first engagement of the contacts 10, 28 or 29 with the common contact connection which is connected to ground. The circuit, shown in FIG. 2, operates as an OR gate in that closure of one or more of the contacts 10, 28 and 29 produces a single output pulse.

It is to be understood that the above-described em bodiments are simply illustrative of the principles of the invention and that many other embodiments may be devised without departing from the spirit and scope of the invention. For example, the polarities of the diodes are arbitrary and anyone skilled in the art could change the polarities of the diodes by making appropriate changes in the biasing voltages. Also, the biasing voltages applied to the various junctions are arbitrary and many other combinations of biasing voltages could be devised by one skilled in the art. The terms charge and discharge" are related to the selected biasing voltages and could easily be changed by changing the biasing voltages. For example, by (1) raising the ground potentials to +20 volts, (2) raising the -3.5 volt potentials to +165 volts, (3) raising the 16 volt potentials to +4 volts and (4) raising the -20 volt potentials to ground potentials, discharging and charging in the second direction would become discharging and discharging and charging in the first direction would become charging. Similarly, all the voltage potentials, including ground, could be lowered 4 volts and then discharging and charging in the second direction would become charging and discharging and charging in the first direction would become discharging.

What is claimed is:

1. A circuit for producing only one output pulse on an output terminal for each closure of a pair of contacts even though the contacts bounce and have several engagements during each closure, comprising:

a diode and a capacitor serially connected between the output terminal and one of the contacts;

means for normally biasing the diode non-conductive;

means responsive to the contacts being open for normally charging the capacitor in a first direction such that when the contacts first engage, the diode is rendered conductive;

said charging means being sufiiciently slow such tha substantially no change occurs in the voltage across the capacitor during an interval between successive engagements of the contacts while they are bouncing; and

means responsive to engagement of the contacts for rapidly discharging the capacitor and charging the capacitor in the second directionduring the first engagement of the contacts during closure such that during subsequent bouncing of the contacts the diode remains non-conductive.

2. A circuit as defined in claim 1, wherein:

the capacitor is connected to the one contact and the diode is connected to the output terminal; and

the charging means in the first direction includes a resistor connected between a voltage source and the junction of the capacitor and one contact.

3. A circuit as defined in claim 1, wherein:

the capacitor is connected to the one contact and the diode is connected to the output terminal; and

the discharging and charging means in the second direction includes a resistor connected between a-voltage source and the junction of the capacitor and diode.

4. A circuit as defined in claim 1, wherein:

the capacitor is connected to the one contact and the diode is connected to the output terminal;

the charging means in the first direction includes a resistor connected between a voltage source and the junction of the capacitor and one contact; and

the discharging and charging means in the second direction includes a resistor connected between a voltage source and the junction of the capacitor and diode.

5. A circuit for producing only one output pulse on an output terminal for each closure of a pair of contacts even though the contacts bounce and have several engagements during each closure, comprising:

a diode and a capacitor serially connected between the output terminal and one of the contacts wherein the diode is connected to the output terminal and the capacitor is connected to the one contact;

a first biasing means connected in parallel with the contacts for applying a voltage to the junction of the one contact and capacitor wherein engagement of the contacts changes the voltage on the junction of the one contact and capacitor;

said first biasing means in conjunction with the capacitor having a first time constant of a duration sufilcient to prevent any substantial change in the voltage on the capacitor in a first direction during an interval between successive engagements of the contacts while they are bouncing;

a second biasing means connected in parallel with the diode for normaly biasing the diode non-conductive and for applying a voltage to the junction of the diode and capacitor; and

said second biasing means in conjunction with the capacitor having a second time constant of a duration such that there is a change in the voltage across the capacitor in the second direction during the first engagement of the contacts for each closure, and the diode is rendered momentarily conductive only during the first engagement to apply a pulse to the output terminal.

-6. A circuit for producing only one output pulse on an output terminal for each closure of a pair of contacts even though the contacts bounce and have several engagements during each closure, comprising:

a diode and a capacitor serially connected between the output terminal and one of the contacts wherein the capacitor is connected to the one contact and the diode is connected to the output terminal;

a resistor and a voltage source serially connected across means for applying a second voltage to the junction of the capacitor and diode, said second voltage applying means in conjunction with the capacitor having a time constant of a duration such that there is a substantial change in the voltage across the capacitor in the second direction during the first engagement of the contacts for each closure; and

means for applying a third voltage to the juncton of the diode and the output terminal such that the diode is normally biased non-conductive and becomes momentarily conductive only during the first engagement of the contacts for each closure to apply a pulse to the output terminal.

7. A circuit as defined in claim 6, wherein: the second voltage applying means includes means for limiting the voltage on the junction of the capacitor and diode between fourth and fifth voltages.

8. A circuit for producing only one output pulse on an output terminal for each closure of one pair of a plurality of pairs of contacts even though the contacts bounce and have several engagements during each closure, comprising:

a plurality of series circuits each including a capacitor and an isolating diode serially connected between a first contact of each pair of contacts and a common junction;

gating diode connected between the common junction and the output terminal;

plurality of resistors each connected in series with a voltage source across a respective pair of contacts for applying a first voltage to a respective junction of each first contact and capacitor such that the engagement of a pair of contacts changes the voltage on the respective junction of a first contact and capacitor, each resistor in conjunction with its respective capacitor having a time constant of a duration sufiicient to prevent any substantial change in the voltage across the respective capacitor in a first direction during an interval between successive engagements of the respective contacts while they are bouncing;

junction, said second voltage applying means in con junction with each capacitor having a time constant of a duration such that there is a substantial change in the voltage across the capacitor in the second direction during the first engagement of the contacts for each closure; and

means for applying a third voltage to the junction of the gating diode and the output terminal such that the gating diode is normally biased non-conductive and becomes momentarily conductive only during the first engagement of a pair of contacts for each closure to apply a pulse to the output terminal.

9. A circuit for producing only one output pulse on an output terminal for each closure of a pair of contacts even though the contacts bounce and have several engagements during each closure, comprising:

a diode and a capacitor serially connected with the output terminal and the contacts;

means for normally biasing the diode non-conductive;

means responsive to the contacts being open for normally charging the capacitor in a first direction such that when the contacts first engage, the diode is rendered conductive;

said charging means having a time constant of sufliciently long duration to be substantially ineffective to change the voltage in a first direction across the capacitor during an interval between successive engagements of the contacts While they are bouncing;

and

means responsive to engagement of the contacts for rapidly discharging and charging the capacitor in the second direction during the first engagement of the contacts during closure such that during subsequent bouncing of the contacts the diode remains non-conductive.

References Cited UNITED STATES PATENTS DONALD D. FORRER, Primary Examiner H. A. DIXON, Assistant Examiner US. Cl. X.R. 307-3l7 

